The Pan and Tilt turrets are servo-controlled positioners provided with to motorized axes, the external axis (or pan axis) being generally an azimuth axis (i.e. a vertical axis when the turret is positioned on an horizontal support), the second axis (or tilt axis) is an elevation axis.
Such turrets are installed on a vehicle, that may be in particular a boat or a plane, but also a terrestrial vehicle. This type of vehicle is liable to perform translational and rotational movements according to the 6 degrees of freedom. In the following of the document, these movements of the vehicle will be qualified as bearing movements. The axes of the turret are motorized. The turret is fixed at its base to the vehicle.
On the internal axis of the turret (tilt axis) is fixed an equipment that may be, for example, a receiver system, or possibly emitter-receiver system, that may notably be of optical or electromagnetic type. This system may be, in particular, a camera or an antenna.
The role of these positioners is to compensate for the bearing movements of the vehicle in such a manner that the line of sight or pointing of the equipment, notably camera or antenna, remains fixed with respect to a local terrestrial reference system. These positioners may be equipped with gyroscopic stabilization means of their own, which will be qualified hereinafter as “intra-positioner”.
Two mains intra-positioner stabilization/servo control-correction methods are known, which are presented now with reference to FIGS. 4 and 5.
It is also known by the document GB2345155A a means for gyro-stabilization of an imager, implementing a Kalman filter. However, the data that are processed in the filter after integration of the measurements of the gyrometers are (angular) position data and, furthermore, corrections are performed on these data before their processing through the Kalman filter. Finally, it is indicated that an additional correction means has to be implemented to obtain a better stabilization.
In order to make easier the following explanations, a number of notations are shown:
0 local terrestrial reference system (north, east, down)
1 reference system associated with the vehicle
2 reference system associated with the turret support
3 reference system associated with the platform supporting the camera
ψ yaw of the vehicle provided by the attitude unit of the vehicle
θ pitch of the vehicle provided by the attitude unit of the vehicle
φ roll of the vehicle provided by the attitude unit of the vehicle
R10 transfer matrix from the reference system  to the reference system .
With:
                              R          10                =                              (                                                            1                                                  0                                                  0                                                                              0                                                                      cos                    ⁢                                                                                  ⁢                    ϕ                                                                                        sin                    ⁢                                                                                  ⁢                    ϕ                                                                                                0                                                                                            -                      sin                                        ⁢                                                                                  ⁢                    ϕ                                                                                        cos                    ⁢                                                                                  ⁢                    ϕ                                                                        )                    ·                      (                                                                                cos                    ⁢                                                                                  ⁢                    θ                                                                    0                                                                                            -                      sin                                        ⁢                                                                                  ⁢                    θ                                                                                                0                                                  1                                                  0                                                                                                  sin                    ⁢                                                                                  ⁢                    θ                                                                    0                                                                      cos                    ⁢                                                                                  ⁢                    θ                                                                        )                    ·                      (                                                                                cos                    ⁢                                                                                  ⁢                    ψ                                                                                        sin                    ⁢                                                                                  ⁢                    ψ                                                                    0                                                                                                                        -                      sin                                        ⁢                                                                                  ⁢                    ψ                                                                                        cos                    ⁢                                                                                  ⁢                    ψ                                                                    0                                                                              0                                                  0                                                  1                                                      )                                              (        1        )            The vehicle is supposed to be equipped with an attitude unit, so the matrix R10 is known.θP angular position of the pan axis of the positioner (axis 1)θT angular position of the tilt axis of the positioner (axis 2){dot over (θ)}p angular rate of the pan axis (axis 1){dot over (θ)}T angular rate of the tilt axis (axis 2)R32 transfer matrix from the reference system 2 to the reference system 3 (transfer from the turret support to the camera support).with:
                              R          32                =                              (                                                                                cos                    ⁢                                                                                  ⁢                                          θ                      T                                                                                        0                                                                                            -                      sin                                        ⁢                                                                                  ⁢                                          θ                      T                                                                                                                    0                                                  1                                                  0                                                                                                  sin                    ⁢                                                                                  ⁢                                          θ                      T                                                                                        0                                                                      cos                    ⁢                                                                                  ⁢                                          θ                      T                                                                                            )                    ·                      (                                                                                cos                    ⁢                                                                                  ⁢                                          θ                      P                                                                                                            sin                    ⁢                                                                                  ⁢                                          θ                      P                                                                                        0                                                                                                                        -                      sin                                        ⁢                                                                                  ⁢                                          θ                      P                                                                                                            cos                    ⁢                                                                                  ⁢                                          θ                      P                                                                                        0                                                                              0                                                  0                                                  1                                                      )                                              (        2        )            R21 Transfer matrix from the reference system 1 (reference system of the vehicle) to the reference system 2 (reference system of the turret support).It is supposed, as a simplification, that R21≈I3 (the turret support has been aligned to the reference system of the vehicle after a calibration process).Let's note that: Rij=RjiT ωx instantaneous rate of rotation of the camera support about the axis xωy instantaneous rate of rotation of the camera support about the axis yωz instantaneous rate of rotation of the camera support about the axis zThe structural scheme of the positioner with its camera is shown in FIG. 2.R30 transfer matrix from the local terrestrial reference system to the reference system of the camera support.with:R30=R32·R21·R10 R30≈R32·R10  (3)q quaternion of the attitude of the camera support with respect to the terrestrial reference system.Let's consider:
  q  =      (                            a                                      b                                      c                                      d                      )  a, b, c, d components of the quaternion. The attitude quaternion is normalized: a2+b2+c2+d2=1R30 may be expressed as a function of ψ, θ, φ, θP, θT, but may also be expressed based on the components of q, i.e.:
                              R          03                =                  (                                                                                          a                    2                                    +                                      b                    2                                    -                                      c                    2                                    -                                      d                    2                                                                                                2                  ⁢                                      (                                          bc                      -                      ad                                        )                                                                                                2                  ⁢                                      (                                          bd                      +                      ac                                        )                                                                                                                        2                  ⁢                                      (                                          bc                      +                      ad                                        )                                                                                                                    a                    2                                    -                                      b                    2                                    +                                      c                    2                                    -                                      d                    2                                                                                                2                  ⁢                                      (                                          cd                      -                      ab                                        )                                                                                                                        2                  ⁢                                      (                                          bd                      -                      ac                                        )                                                                                                2                  ⁢                                      (                                          cd                      +                      ab                                        )                                                                                                                    a                    2                                    -                                      b                    2                                    -                                      c                    2                                    +                                      d                    2                                                                                )                                    (        4        )            with: R03=R30T  (5)
The pointing vector (line of sight of the camera) is the vector X of the reference system . This pointing vector (unitary vector) may be expressed with respect to the local terrestrial reference system by means of an azimuth and elevation component.
θaz azimuth angle of the pointing vector,
θel elevation angle of the pointing vector.
FIG. 3 gives a diagram showing the relations between the terrestrial reference system and the pointing vector.
Through handling of the transfer matrices, the following relation is shown:
                              (                                                                      cos                  ⁢                                                                          ⁢                                                            θ                      el                                        ·                    cos                                    ⁢                                                                          ⁢                                      θ                    az                                                                                                                        cos                  ⁢                                                                          ⁢                                                            θ                      el                                        ·                    sin                                    ⁢                                                                          ⁢                                      θ                    az                                                                                                                                            -                    sin                                    ⁢                                                                          ⁢                                      θ                    el                                                                                )                =                              R            03                    ·                      (                                                            1                                                                              0                                                                              0                                                      )                                              (        6        )            
After these annotations, the known techniques of stabilization will be described.
The objective of the gyro-stabilization is to keep constant the line of sight of the equipment carried by the positioner with respect to the terrestrial reference system, despite the bearing movements of the vehicle.
That is to say that it is searched to keep constant the azimuth and the elevation of the pointing vector: θax and θel.
The pan and tilt axes of the turret are controlled by electric motors, generally DC brushless motors whose torque is proportional to the current command they receive. These motors/axes generally have angle encoders that allow knowing the angular position of the controlled axes.
The state of the art essentially includes two methods of stabilization.
The first method shown in FIG. 4 consists in making, on each of the pan and tilt axes, angular position servo controls, the controlled axes including angle encoders.
Let's call:
θP* the command on the pan axis,
θT* the command on the tilt axis,
θaz* the command of the pointing vector azimuth,
θel* the command of the pointing vector elevation,
the control law consists in calculating θP* and θT* from the commands θaz* and θel*, the matrix R10 being known because given by the attitude unit of the vehicle.
The (single-variable) servo control of the pan and tilt axes presents no difficulty and the structure of the control law is shown in FIG. 4.
This first method is simple to implement but results in a rather poor quality of stabilization for several reasons:
The hypothesis has been made that R21=I3, resulting from a process of alignment of the turret support with respect to the reference system of the vehicle. This alignment is never perfect and there results therefrom a deteriorated pointing stability.
The physical link between the attitude unit and the positioner is not necessarily rigid or stable because the positioner may be installed at a rather great distance from the attitude unit (10 m, for example), on a support (for example, a post) that is a little flexible with respect to the reference system of the vehicle and the link variations cause a deterioration of the correction.
The “bandwidth” of the unit is generally rather small: the disturbing movements whose frequency is higher than a few Hertz are not perceived by the unit, and a fortiori are not corrected.
The dynamics of command tracking of the position servo controls may also be an obstacle if it is too low.
To try to remedy these drawbacks, it has been proposed to install the attitude unit on the positioner's base and to use the information coming from the latter to calculate the position commands for the Pan and Tilt axes. This variant gives better results because the bearing movements affecting the positioner are directly measured. It has a major drawback, which is of economic nature: the over-cost generated by the unit may then be very high.
The second method shown in FIG. 5 consists in implementing a local, intra-positioner correction, at the positioner. For that purpose, two gyrometers are placed on the axes Z and Y of the camera support (located the closest to the latter) in order to measure the instantaneous rates of rotation ωy, ωz.
Two rate servo controls are then performed in order to maintain the rate commands ωy*, ωz* such that ωy*=0, ωz*=0.
In the particular case where the support of the positioner is in fixed position on the vehicle:
                                          (                                                                                ω                    y                                                                                                                    ω                    z                                                                        )                    =                                    (                                                                    1                                                        0                                                                                        0                                                                              cos                      ⁢                                                                                          ⁢                                              θ                        T                                                                                                        )                        ·                          (                                                                                                                  θ                        .                                            T                                                                                                                                                          θ                        .                                            P                                                                                  )                                      ⁢                                  ⁢                              (                                                                                ω                    y                                                                                                                    ω                    z                                                                        )                    =                      J            ·                          (                                                                                                                  θ                        .                                            T                                                                                                                                                          θ                        .                                            P                                                                                  )                                                          (        7        )            where J is the Jacobian matrix of the positioner, which is singular for:
      θ    T    =      ±          π      2      
The corresponding control law is shown in FIG. 5, where Cp and Ct are the rate correctors on the pan and tilt axes, respectively. The correctors Cp and Ct may be synthesized according to methods that are known in the field, whether they are conventional (PID: “Proportional Integral Derivative”), or more sophisticated, in particular (LQG (Linear Quadratic Gaussian control) H∞ (Hinfinite) or pole placement) and that will not be detained herein.
This second method has the interest to regulate the rates at the closest of the camera: It results therefrom a better quality of the image stabilization. It also allows rejecting disturbances at far higher frequencies than in the first method. It is thus, from this point of view, preferable to the first method.
However, this second method has a major drawback because the gyrometers have a bias, constant in first approximation, which causes a drift on θaz and θel and thus on the pointing when the rate servo controls are operational.
The performances of the second method thus strongly depend on the quality of the gyrometers used. But, the more the bias of the gyrometers is reduced, the more their bulk is great, and their cost is high. By way of example, the following table gives the over-cost for three gyrometers as a function of the bias level.
BIAS GYRO (°/s)COST (dihedral) (   ) 10°/s 15    1°/s150  0.1°/s1500   
It is desirable to implement means that allow a stabilization of the positioner of quality for a reduced cost. But in some cases, the vehicle may originally include an attitude unit giving measurements of attitude of the vehicle and the use of these measurements could also be contemplated for improving the stabilization of the own, intra-positioner, means of stabilization of the positioner. In other cases, the vehicle does not include originally an attitude unit and the use of measurements of attitude of the vehicle requires the installation of an attitude unit on the vehicle, whose cost may still be far higher for precise measurements.